The underlying assumption of this proposal is that T cell receptors are much like immunoglobulin heavy chains in their structural composition; T and B cells probably share V region gene products, but the constant segments of T cell receptors are probably antigenically distinct from the known immunoglobulin class markers. That T cell receptors have V regions identical with B cells has been established using anti-immunoglobulin class markers. That T cell receptors have V regions identical with B cells has been established using anti-immunoglobulin idiotype serum to block T cell functional assays. The receptors extracted from T cells which have binding specificity for antigen are too large (100,000 M.W.) to be dimers of V regions alone and probably represent a V region attached to another gene product, i.e., a T cell constant region gene product. An attempt will be made to make antisera directed against constant markers on T cell receptors for defiend antigens. Specifically, congenic mice will be used to look for idiotypic markers on Fv regions and allotypic markers on C region determinants. Heterologous antisera will be used to detect class specific markers. The objective is to (a) investigate whether there are V and C region gene products of the same molecule and (b) to compare the inheritance patterns of T and B cell allotypes. For example, B cell allotypes fall into linkage groups which are unrelated to H-2 types; with the T cell allotypic markers parallel the B cell linkage groups, or are the genes which code for T and B cell constant regions unlinked? If an antiserum to either an idiotypic or an allotypic determinants is developed, it will be used to precipitate radiolabeled complexes of cells for peptide mapping studies to see if the molecule has chemically heterogeneous and homogeneous segments. In addition, major emphasis will be given to use of any antisera developed to study the differentiational pathway of a T cell. It is possible that a T cell may express V region gene products linked to different C gene products at the cell surface in different stages of development. Alternatively, the T cell receptors for different functional subsets, i.e., helper, suppressor, or alloreactive cell, may represent cells with different constant-regions analogous to the immunoglobulin markers.